1. FIELD OF THE INVENTION
The present invention relates to a multi-area(section) brightness measuring apparatus for obtaining an optimum exposure value for an automatic exposure control.
2. DESCRIPTION OF RELATED ART
An automatic exposure control device, which enables a photographer to easily take a picture, is incorporated in almost all of conventional cameras. The exposure can be automatically determined by a combination of an aperture and a shutter speed related to the luminance of an object to be photographed. It is necessary to detect an optimum exposure value (Ev) depending on a luminance of an object in order to effect such an automatic exposure control. At this end, a conventional camera usually has a brightness measuring device (photometer) which is a reflected-light exposure meter. However, the exposure value detected by the reflected-light exposure meter largely depends on a reflectance of the object and a measuring area of the object to be metered etc. Namely, in case of an object including a light sky or a white background, an exposure of a main object is an under-exposure. In particular, an extreme back light leads to an intensive under-exposure. On the other hand, in case of an object including a black or dark background, an exposure of the main object is an over-exposure.
The solution to the drawbacks mentioned above is disclosed for example in Japanese Unexamined Patent Publication (Kokai) No. 62-203141 in which an exposure meter has a brightness measuring means which has, at a center portion of a picture plane, a center brightness measuring area including a center of a picture plane and a plurality of adjacent brightness measuring areas including a plurality of concentrically divided adjacent areas provided on the circumference of the center brightness measuring area. When the size of an image of the main objects in the center brightness measuring area is detected by a magnification of the image calculated by detected object distance and focal length information of the taking lens, an exposure correcting value is determined in accordance with a relationship of brightness between the main object and the background, obtained by the output signals of the center brightness measuring area and the adjacent brightness measuring areas concentrically located in respect to the center brightness measuring area, so that the exposure correcting value thus obtained is added to a center weighted averaged overall light reading value which is obtained by the outputs of the center and adjacent brightness measuring areas to determine an optimum exposure value.
In the multi-area brightness measuring apparatus mentioned above, it is possible to suppose a brightness measuring area to which the object corresponds to obtain the exposure correcting value based on an inclination of the brightness of the brightness measuring areas corresponding to the object, thus resulting in an optimum exposure value.
However, the provision of a plurality of brightness measuring areas makes the known exposure meter complex. Furthermore, the known exposure meter must perform the complicated detection and calculation processes of the center weighted averaged overall light reading value because it has many brightness measuring areas and sensors therein. In addition, since the exposure correcting value is detected on the basis of the relationship of brightness of the image of the main object and the background only when the size of the image of the main object corresponds to the center brightness measuring area, the exposure value can not be corrected when the size of the image of the main object does not correspond to the center brightness measuring area.